1. Field of the Invention
The invention pertains to the field of tensioners used in chain drive power transmission systems. More particularly, the invention pertains to the use of a resilient pad in combination with an optional pre-loaded spring under a chain sliding face to carefully regulate spikes or excessive surges in chain tension.
2. Description of Related Art
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission devices, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that at any point in time, part of the chain is slack and part of the chain is tight. In the case of a chain engaging toothed sprockets, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or the unmeshing of teeth from any of the sprockets. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because the jumping of teeth will throw off the camshaft timing, causing damage to the power transmission system or possibly rendering the engine inoperative.
However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in chain tension. For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example during stopping of the engine or in failed attempts at starting, can also cause extreme spikes or surges in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure that adequate tension is provided on the slack side of the chain.
Blade tensioners are tensioners may be used to control a chain or belt where load fluctuations are not so severe as to excessively flex the spring or springs. A ratchet with backlash may be added to such tensioners to limit the effective backward or untensioned travel of the tensioning device.
Referring to FIG. 1 an example of a typical blade tensioner is shown. The blade tensioner 110 includes a blade shoe 111 made of resin having a curved chain sliding face and, in this example, numerous blade springs 121, preferably all made of a metallic material. The blade springs 121 are arranged in layers on the opposite side of the blade shoe 111 from the chain sliding face, and provide spring force to the blade shoe 111. The ends of each blade spring 121 are inserted in the indented portions 114 and 115, which are formed in the distal portion 112 and proximal portion 113 of the blade shoe 111, respectively. A bracket 117 is provided for mounting the blade tensioner 110 in an engine. Holes 118 and 119 are formed in the bracket 117, and mounting bolts are inserted into these holes 118 and 119. A sliding face 116 contacts the distal portion of the blade shoe 111 and permits sliding. The slide face 116 is formed on the distal portion of the bracket 117. A pin 120 supports the proximal portion 113 of the blade shoe 111 so that it may move in either direction. The pin 120 is secured on the bracket 117.
FIG. 2 shows a chain tensioning device that has a pair of arms 202, 203 which are joined by a pivot 204. The arms 202, 203 are urged apart so that arm 203 applies tensioning force to a chain (not shown) by means of a spring 206 loaded cam block 205. To prevent collapse of arm 203 during load reversals of the chain, a catch disc 209 and rod 208 are arranged to prevent the return movement of the spring loaded cam block 205.
FIG. 3 shows an example of a tensioner with a ratchet device. The ratchet tensioner 301 comprises a tensioner housing 307 having a hole 312 for receiving a piston 308 and a ratchet pawl 317 pivoted by a shaft 316 to the tensioner housing 307 and biased by a ratchet spring 318. The piston 308 has teeth on one outer side that engage a tooth shaped tip of the ratchet pawl 317. The piston 308 is biased out of the hole 312 to contact the tension lever 310 by the combination of adding pressurized hydraulic fluid into the hollow section 313 and the biasing force of the plunger spring 314. The tensioner lever 310 pivots on support shaft 309 and has a shoe surface 311 that contacts and applies tension to the slack side of the timing chain 306 which operatively connects the sprocket 305 of the camshaft 304 with the sprocket 303 of the crankshaft 302. The movement of the piston 308 in and out of the hole 312 is limited by the engagement of the teeth of the ratchet.
Hydraulic tensioners may also be used to maintain proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain when the chain is slack, but must remain rigid when the chain tightens.
To achieve this objective, a hydraulic tensioner 1, as shown in prior art FIG. 4, typically comprises a rod or cylinder as a piston 2, which is biased in the direction of the lever arm by a tensioner spring 3. The piston 2 is housed within a cylindrical housing 5, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing contains a pressure chamber 4 in connection with a reservoir or exterior source of pressurized hydraulic fluid. The pressure chamber 4 is typically formed between the housing 5 and the piston 2, and it expands or contracts as the piston 2 moves within the housing 5.
Typically, valves are employed to regulate the flow of hydraulic fluid into and Out of the pressure chamber. For instance, an inlet check valve such as a ball-check valve opens to permit fluid flow into the pressure chamber 4 when the pressure inside the chamber has decreased as a result of the outward movement of the piston 2. When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber. The closing of the inlet check valve prevents the piston chamber from releasing hydraulic fluid, which in turn prevents the piston from retracting, achieving a so-called “no-return” function.
Many tensioners also employ a pressure relief mechanism that allows fluid to exit the pressure chamber at a controlled rate when the pressure in the chamber is high, thus allowing the piston to retract in response to rapid increases in chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve. The check valve opens when the pressure exceeds a certain pressure point. Some tensioners may employ a valve which performs both the inlet check function as well as the pressure relief function.
Other mechanisms employ a restricted path through which fluid may exit the fluid chamber, such that the volume of flow exiting the fluid chamber is minimal unless the pressure in the fluid chamber is great. For instance, a restricted path may be provided through the clearance between the piston and bore, through a vent tube in the protruding end of the piston, or through a vent member between the fluid chamber and the fluid reservoir.
U.S. Pat. No. 5,993,342 discloses a chain tensioner of the hydraulic piston type having an additional feature. The piston provides a tension limiting force on a pivoting lever or arm that is in slideable engagement with a strand of the chain. At the point of contact between the piston and the lever or arm, the end of the piston contains a preloaded compression spring and, optionally, an elastomeric disc or cushion to help control sudden spikes or surges in chain tension. One of the limitations associated with the use of such designs is the increasing amount of space that is needed within the compartment that houses the drive transmission system. This is as the result of the extended length of the piston due to the spring and elastomeric disc added to its contact end.
It is desirable to maintain a substantially constant tension on the chain. It is especially beneficial with respect to the issue of chain wear if the required tensioning forces can be minimized during the times that the engine is off. In the prior art devices detailed above, the hydraulic tensioning systems address this problem by permitting the slow leakage of hydraulic fluid when the engine is shut off thereby allowing the chain tension to slacken gradually. The ratcheting systems address this issue by the inherent presence of backlash in such systems. However, due to design limitations or cost restrictions, backlash may be undesirable or unattainable for certain power transmission systems.